1. Field of the Invention
The invention relates to the field of mobile communications networks, to be more precise to managing the transmission of digital data blocks in HS-DSCH transport channels of such networks.
2. Description of Related Art
As the person skilled in the art is aware, the fifth version of the 3GPP specifications governing the transmission of multimedia data in mobile (or cellular) communications networks, for example GSM/GPRS, UMTS and EDGE networks, proposes a High Speed Downlink Packet Access (HSDPA) mechanism, one particular objective of which is to provide a high bit rate for transmitting downlink packet traffic, for example Internet traffic, that is virtually insensitive to transmission delay. It is based on using transport channels known as high speed—downlink shared channels (HS-DSCH) that are shared between user equipments.
An HS-DSCH transport channel is situated between the MAC layer and the physical layer. Data bits coming from the HS-DSCH transport channel are transmitted to a user equipment in a set of physical channels that is situated at the level of the physical layer and that consists of at least one physical channel called an HS-PDSCH.
In the present context, the expression “user equipment” means any communications terminal able to exchange data by radio with a mobile (or cellular) communications network.
Obtaining high bit rates relies in particular on employing a Hybrid Automatic Repeat reQuest (HARQ) technique whereby, each time that a user equipment receives an incorrect data block from its base station on an HS-DSCH transport channel, that base station is sent a Non-Acknowledgment (NACK) message indicating incorrect reception, as a result of which it sends it a new block of data bits in the HS-DSCH transport channel. On receiving this new block, the user equipment combines the data that it contains with the data contained in the block or blocks previously received incorrectly, which is temporarily stored in a dedicated buffer.
There are two retransmission modes, one called “chase combining” (CC) and the other called “incremental redundancy” (IR). On each occasion, the CC mode retransmits a block containing the same set of data as the original block, whereas the IR mode retransmits blocks containing sets of data that all differ partly from each other. In the IR mode, each set of data is chosen as a function of the value of a redundancy parameter XRV that varies from one block to another and is used in a rate matching step.
As the person skilled in the art is aware, the IR management mode is more efficient than the CC management mode, so that it is particularly important to select optimum values of the redundancy parameter XRV.